First, welcome to the Maple Trader.
On your question I have some questions. A comment first, don't try to put the taps on any particular side. I used to train my workers on new trees being tapped, dril on the side you approach from, don't look for a southern exposure, because if you always try for the southern exposure you will soon run out of tapable wood. First question, what size bit are you using? Is it a maple specific bit or is it a common hardware store bit? How deep do you drill, in past the bark? Do the shavings come out white or are they stained darker? These are all important. If the drill shavings are stained, you drilled into a previously tapped spot. Once a tree is tapped, it seals off that area, because it was losing sap, it's a normal defensive process. If the shavings were white and you drilled using a maple drill bit (they cost more but are worth the difference. Also, don't use that bit to drill anything but tap holes. Save it, boil it after you finish, then once completely dry put it in a zip lock bag with a label telling everyone that it's only to be used to tap maple trees. Did you drill 1.5-2" into the wood, once past the bark.
Then next year look for last years healed up tap hole, move rt or left 3" and up or down 6" That will keep you away from any compartmentalization (that's how a tree seals off a previous tap hole or damaged area.) Continue which ever way you moved from the previous hole every year.
Now, on a tree that is 3-4' diameter (not circumferance) when on gravity ( buckets, bags or drop tubing into a container on the ground you can have a tap every 30" in circumfrance, but never more than 3 total and if the 3rd one doesn't leave at least 30" to the first one, you're best off havinf one tap less. In Diameter a 10" tree can get 1 tap, a 20" tree can get 2 taps and a 30" and up can get 3 taps. That changes if you ever get vacuum, even 3/16 tubing with good fall in elevation gets gravity vacuum. If you ever get vacuum, 10" 1 tap, 20" and up 2 taps. Put the taps on the opposite side of the tree from the 1st tap.
What dates did you tap aproximate only? How did the rest of the trees flow? bIf you used a inferior drill bit (hardware store for steel, or maybe not very sharp, that can cause poor or no flow. Use a proper maple bit, drill in the prescribed depth (it helps to put a little piece of 5/16 tubing on the bit as a marker for depth.) A good, very sharp bit will yield greater sap flow and for longer once tapped.
These are all I can think of at this time, but don't give up. You will find the answer in time. If you do hit stained wood, it's best to skip the tap, you've already wounded the tre, don't mave and make another wound. On a younger tree, once you work your way around a tree using the 3" over and 6" up or down, by the time you get back to the first tap hole the tree will have added enough new wood to drill over the old healed spot, but best only 1.5" in past the inside of the bark.
Good luck and most of all, have fun. At some point you will likely go with an official evaporator, likely wood fired, that will bring the fuel cost way down.
Dave Klish, I recently bought a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.